Calcium sulphate-based products are widely used in the construction of buildings, for example, to form internal partitions (using wallboard, also known as dry wall, gypsum board or plaster board) and ceilings or to encase ducts (e.g. ventilation ducts) within buildings.
Calcium sulphate-based products such as wallboard are typically formed by drying an aqueous slurry of the hemihydrate of calcium sulphate (CaSO4. 1/2 H2O), also known as calcined gypsum or stucco, between two sheets of lining paper or fibreglass matting. As the slurry dries and the calcined gypsum is hydrated, a hard, rigid core of gypsum (calcium sulphate dehydrate—(CaSO4.2H2O)) sandwiched between the lining sheets/mats is formed.
When wallboard or ceiling tiles are exposed to high temperatures such as those experienced in a building fire or those experienced by wallboards used for encasing ducts carrying high temperature fluids, the water of crystallization contained within the gypsum is driven off to yield the anhydrite of calcium sulphate. Initially, this has the advantage that heat transfer across the wallboard/ceiling tile is reduced thus helping to contain the heat emanating from ducting or generated during a building fire. However, at temperatures around 400-450° C., the initially formed AIII phase anhydrite (also known as γ-CaSO4 or “soluble” anhydrite) converts to the AII phase (or “insoluble” anhydrite) and this phase change results in shrinkage of the wallboard/tile i.e. a loss of dimensional stability. This shrinkage (which may be around 2% of the wallboard's/tile's length or width, or around 6% of the wallboard's volume) often causes the wallboards to pull away from their supporting structures. This is obviously undesirable. It can leave ducts exposed to high temperatures. Furthermore, in situations where wallboard is used for internal partitions and a fire breaks out, shrinkage can leaves gaps exposing rooms adjacent to the fire source to the effects of the heat/fire. Gaps also allow ingress of oxygen into the fire source thus fuelling the fire and negating the effects of any fire doors.
At higher temperatures (in excess of 600° C.), the insoluble anhydrite goes on to sinter resulting in large reductions in wallboard volume. This results in extreme shrinkage which eventually causes collapse of the internal walls/ceilings/duct casings as they are no longer held by their supporting structures.
Efforts have been made to improve the heat resistance of calcium sulphate-based products such as wallboard in an attempt to reduce shrinkage.
It is known e.g. from EP0258064 to use micro silica as an additive in the gypsum core of wallboard to reduce shrinkage.
However, these additives only have an effect at temperatures greater than 600° C. i.e. they do not resist the shrinkage of the board at lower temperatures and linear shrinkage of more than 10% is still seen as temperatures around 1000° C.
It is known from WO99/08979 and WO00/06518 to add sodium trimetaphosphate (STMP), sodium hexametaphosphate (SHMP) or ammonium polyphosphate (APP) to a calcium sulphate wallboard core to improve strength, sag resistance and shrinkage during drying. No effect of these additives on shrinkage during exposure to high temperatures is recorded. The trimetaphosphate ions and APP were found to accelerate the rate of hydration of calcined gypsum thus decreasing the set time for the wallboard core.
WO2012/069826 discloses use of aluminium and ammonium phosphate additives for enhancing fire resistance of calcium sulphate-based products. Ammonium polyphosphate (APP) was found to reduce hydration time of the calcined gypsum and accelerate setting time.
Calcium sulphate-based products are also used to cast metal objects. Calcium sulphate moulds are heated to 700-900° C. prior to being filled with molten metal. It is important to control high temperature shrinkage of such calcium sulphate-based moulds to ensure that the moulds do not leak and to ensure that the cast metal products are not warped.
A preferred aim of the present invention is to provide an improved heat resistant calcium sulphate-based product having reduced shrinkage after heat exposure e.g. when in contact with ducting, during a building fire or during casting of metal products. Such an improved heat resistant product may have particular use as wallboard or panels for forming internal partitions in buildings, ceiling tiles, wallboard or panels for encasing ventilation/smoke extraction ducting, joint filler materials for joining wallboard/panels/tiles or for moulds for use in metal product casting.